Power supply device

ABSTRACT

The present invention relates to a power supply apparatus, more particularly to an improvement of a power supply apparatus provided with a plurality of secondary batteries connected serially, as well as a control of charging and discharging of the secondary batteries to suppress an accelerative deterioration of the secondary battery. The present invention provides to a control unit with a function for stopping the charging when the charged amount in charging becomes larger than the predetermined value, another function for changing the initial set value to a larger value when a variance in the charging capacity among the secondary batteries in charging becomes larger than the predetermined value.

TECHNICAL FIELD

The present invention relates to a power supply apparatus, particularlyto a power supply apparatus comprising a plurality of secondarybatteries connected serially, and more concretely to a control ofcharging and discharging in such power supply apparatus, to eliminate avariance in capacity among the batteries and to prevent an accelerativedeterioration of the batteries.

BACKGROUND ART

Various power supply apparatuses in which many secondary batteries suchas alkaline secondary batteries and nickel-metal hydride secondarybatteries are connected serially have been proposed. Such power supplyapparatuses are now attracting a great deal of attention especially asdriving power sources of electric vehicles. Since a high voltage of200-300 V is required as a driving power source of an electric vehicle,each of such power supply apparatuses is provided with a battery unitincluding about 200 secondary batteries connected serially. In such apower supply apparatus, the battery unit is charged by an external powersource and discharged to an external load such as a motor. Usually, theelectrical power unit is provided with monitor and control means formonitoring a state of each battery to prevent both overcharging andoverdischarging. The monitor and control means stops the charging whenthe battery unit voltage reaches a predetermined upper limit thereby toprevent overcharging in charging, and the monitor and control meansstops the discharging when the battery unit voltage drops to apredetermined lower limit thereby to prevent overdischarging indischarging.

Regardless of the battery type, a deterioration of a secondary batteryincluding a decrease in battery capacity and an increase in internalresistance thereof progresses acceleratively as charge/discharge cyclesare repeated. Further, if many secondary batteries are used, thedeterioration of battery badly affects other batteries. According to thecontrol as described above, for example, a secondary battery whosecapacity is reduced due to the deterioration is further deterioratedsince it is apt to be overdischarged. In addition, heat due to theincrease in internal resistance of the secondary battery causes a dropin output or deterioration of other secondary batteries. Also a normalsecondary battery deteriorates and generates heat thereby increasing thesurrounding temperature and even when charged fully, may be overcharged,if the charging efficiency of the secondary battery around it drops.

If many batteries are to be used, a temperature difference amongbatteries, for example, causes the deterioration of batteries. Inaddition to the facilitation of the deterioration due to hightemperature, a difference in charging efficiency or self-dischargecharacteristics among the batteries caused by a temperature differencealso causes the overcharging or the overdischarging of the batteries.Further, even when every secondary battery is used under the samecondition, a difference in original characteristics among respectivebatteries such as capacity, charging efficiency and self-dischargingcharacteristics also becomes a factor is the deterioration.

Therefore, various means to suppress such accelerative deterioration inbattery characteristics have been proposed. For example, the OfficialGazette of Unexamined Japanese Patent Publication No. Hei 6-231805 hasproposed a method in which a voltage of a battery unit composed of aplurality of batteries connected serially is monitored and a voltage ofeach battery is also monitored, then a charge upper limit voltage iscorrected to be lower and a discharge lower limit voltage is correctedto be higher when a variance in battery voltage becomes large.

As shown in FIG. 4, a power supply apparatus 30 comprises a battery unit32 for supplying an electric power to a load 33 such as a motor, andcontrol and monitor means 34 for controlling and monitoring the batteryunit 32.

A charging operation of the power supply apparatus proposed in theabove-mentioned gazette will be explained with reference to a flow chartshown in FIG. 5. In charging, the monitor and control means 34 controlscharging of the battery unit 32 performed by a power source 31 accordingto a state of the battery unit 32 while monitoring the state of thebattery unit 32. When the charging is started, the monitor and controlmeans 34 makes the power source 31 supply a current to the battery unit32 while monitoring a voltage of each battery included in the batteryunit 32. In step 301, the monitor and control means 34 compares thevariance (σ₀) in voltage among the batteries with a preset value (D₀).At this time, if the σ₀. is smaller than the D₀, the monitor and controlmeans 34 judges the battery unit 32 is being normal and keeps the supplyof current from the power source 31 to the battery unit 32 until thecharged amount (C) of the battery unit 32 reaches a predetermined upperlimit capacity (C_(u)) (in step 302). On the other hand, if the σ₀ isnot smaller than the D₀, the monitor and control means 34 changes thecharging amount used to stop charging from C_(u) to a corrected upperlimit capacity (C_(uc)) which is smaller than C_(u) (in step 303). Atthis time, the battery unit 32 is charged up to the C_(uc), then thecharging is finished.

According to this method, the accelerative deterioration of the batterycan be prevented, but this method includes a problem wherein the fullperformance of the battery is not brought out since batteries with nodeterioration are not charged fully.

Next, a discharging operation of this power supply apparatus will bedescribed with reference to a flow chart in FIG. 6. When discharging isstarted, the monitor and control means 34 supplies an electric powerfrom the battery unit 32 to the load 33 while monitoring a voltage ofeach battery. At this time, it compares a variation (σ₁) in the voltageamong batteries with a preset value (D₁) in step 401. If the σ₁ issmaller than the D₁, the monitor and control means 34 judges the batteryunit 32 as being normal and keeps the battery unit 32 discharged to thelower limit capacity (C₁) that is the original discharging lower limit(step 402), then stops the discharging. If the σ₁ is over the D₁,however, the monitor and control means 34 changes the capacity value atwhich the discharging is stopped from the C₁ to a corrected lower limitcapacity (C_(1c)) which is larger than the C₁ (in step 403). Andaccordingly, the battery unit 32 is discharged to C_(1c) and thedischarging is finished.

According to this method, it is possible to prevent an accelerativedeterioration of batteries as same in a charging operation, but it isimpossible to carry out the full performance of the battery. Inaddition, since it is impossible to eliminate a memory effect of abattery whose performance has dropped, the variance in capacity amongthe batteries cannot be eliminated substantially.

DISCLOSURE OF INVENTION

The object of the present invention is to solve the above-mentionedproblems and to provide a power supply apparatus which can bring out afull performance of a battery as well as eliminating an accelerativedeterioration of the battery.

A power supply apparatus of the present invention includes a batteryunit for supplying an electric power to a load, and a control unit forstopping charging if the charged amount of the battery unit becomeshigher than a preset value, wherein the battery unit comprises aplurality of batteries connected serially, and the control unit has afunction for changing the preset value of the charging amount of thebattery unit to a larger value if a variance in charging capacity amongthe secondary batteries becomes large in charging.

By increasing the charging amount, a battery which is insufficientlycharged due to decline in charging characteristics is brought into thealmost full-charged state because of the further charging. Thus, thevariance in capacity among the batteries is corrected. Further, sinceeach normal battery is charged sufficiently, the full capacity of thebattery unit can be brought out. Consequently, it is also possible toprevent a sharp drop of an output voltage of the power supply apparatusdue to the decrease in charging amount of each batteries. Therefore, itbecomes possible to provide a highly reliable power supply apparatuswhich can stably supply an electric power.

In such a case, a rise in inner pressure of each battery in charging canbe suppressed if the charging rate is set to 0.3 C or lower. Especially,when the object secondary battery is a nickel-metal hydride storagebattery, a significant effect can be obtained since monitoring the stateof the battery becomes easy.

Another power supply apparatus of the present invention includes abattery unit for supplying an electric power to a load, and a controlunit for stopping discharging if the discharged amount of the batteryunit becomes larger than a preset value, wherein the battery unitcomprises a plurality of secondary batteries connected serially, and thecontrol unit comprises a function for changing the preset value of thedischarging amount of the battery unit to a larger value if a variancein remaining capacity among the secondary batteries in dischargingbecomes large.

In discharging, if the variance in remaining capacity increases, thedischarge lower limit voltage is made lower. Consequently, the memoryeffect is eliminated from the battery whose capacity has dropped, sothat the capacity is restored. Therefore, it becomes possible to carryout the full performance of the battery.

The preset value of such charging or discharging amount is changed whenthe variance monitored by the control unit becomes higher than apredetermined value.

In the same way, it is also effective to change the preset value whenthe number of charging or discharging cycles reaches a predeterminedcount decided by considering the cycle life of each battery.

In addition, it is further effective to change the preset value in thesame way when the battery unit is left in a state of being fully chargedor discharged for a predetermined period or longer.

The charging capacity or remaining capacity described above can be knownby measuring the voltage of the object battery without measuring itdirectly.

As described above, according to the present invention, the heat ordeterioration in charging or discharging of the battery can besuppressed since the variance in capacity among batteries can besuppressed. Therefore, the present invention can significantly improvedue reliability of a power supply apparatus. The effect of the presentinvention will be more marked especially when the power supply apparatusis used for an electric vehicle or the like which needs an output withlarge capacity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of a power supplyapparatus in an embodiment of the present invention.

FIG. 2 is a flow chart indicating a charging control of the same powersupply apparatus.

FIG. 3 is a flow chart indicating a discharging control of the samepower supply apparatus.

FIG. 4 is a block diagram for a configuration of a prior art powersupply apparatus.

FIG. 5 is a flow chart indicating a charging control of the same powersupply apparatus.

FIG. 6 is a flow chart indicating a discharging control of the samepower supply apparatus.

BEST MODE FOR CARRYING OUT THE PRESENT INVENTION

A preferred embodiment of a power supply apparatus of the presentinvention are shown in FIG. 1. A power supply apparatus 20 includes abattery unit 2 as an output source and a control unit 4 for monitoringand controlling a charging and discharging of a battery unit 2. Thebattery unit 2 stores an electric power supplied from a power source 1and supplies it to a load 3. For example, if this power supply apparatusis employed as a driving power source for an electric vehicle, the load3 is a motor.

The battery unit 2 includes a plurality of secondary batteries 5connected serially, and a charging circuit 12 and a discharging circuit13 for control charging and discharging of the secondary batteries 5,respectively. As the secondary batteries 5, for example, nickel-metalhydride secondary batteries, alkaline secondary batteries or lead acidsecondary batteries are employed. In the battery unit 2, the secondarybatteries 5 are divided into groups each includes a fixed number of themconnected serially, thereby forming a plurality of units 16-19.

The control unit 4 includes voltage detecting circuits 6-9 for detectingvoltages of the units 16-19, respectively, and a full-charge detectingcircuit 11 for detecting the total voltage of the secondary batteries 5.The control unit 4 further includes a control circuit 10 for outputtingcontrol signals to the charging circuit 12 and discharging circuit 13based on the signals detected by the voltage detecting circuits 6-9 andfull-charge detecting circuit 11.

Hereunder, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

Embodiment 1

In this embodiment, an operation of the above-mentioned power supplyapparatus at charging will be described.

An operation for controlling a charging will be explained using a flowchart shown in FIG. 2.

When a current supplied from the power source 1 to the battery unit 2and the charging is started, the full-charge detecting circuit 11detects the total voltage of the secondary batteries 5 and judgeswhether the secondary batteries 5 are charged up to the upper limitcapacity or a normal full-charging value based on the detected value.

In step 101, the full-charging detecting circuit 11 compares the totalcharged amount (C) of the secondary batteries 5 with the upper limitcapacity (C_(c)) and if the C does not reach the C_(c) value yet, thefull-charge detecting circuit 11 continues charging.

When the C value reaches the C_(c) value, each of the voltage detectingcircuits 6-9 detects the charged amount of the corresponding unit andoutputs a signal to the control circuit 10 based on the detected value.The control circuit 10 compares the signals from the voltage detectingcircuits 6-9 to calculate a variance (e.g., standard deviation) involtage among the units. The control circuit 10 further compares thevariance (σ_(c)) in voltage among the units 16-19 with a preset valueD_(c). If the σ_(c) value is the D_(c) value or lower, the controlcircuit 10 judges that normal charging occurred and outputs a signal forfinishing the charging to the charging circuit 12. The charging is thusfinished normally. However, if the σ_(c) value is over the D_(c) value,the control circuit 10 replaces the C_(c) with a corrected upper limitcapacity (C_(cc)) which is larger than the C_(c) and continues thecharging. If the control circuit 10 judges the C value to be reached toC_(cc), the charging is finished.

That is, the battery which is not fully charged due to a low chargingefficiency thereof is further charged thereby to be brought into anearly full-charged state. In such a manner, the variance in chargingcapacity among batteries can thus be minimized, and the full performanceof the battery can be brought out, although a larger capacity batteryare closely overcharged in comparison with a smaller capacity battery.

The charging rate should preferably be 0.3 C or lower in order tosuppress a raise in pressure of the battery.

Although the corrected upper limit capacity C_(cc) is set larger by 30%than the upper limit capacity C_(c) in the above embodiment, settingC_(cc) larger may cause overcharge. On the other hand, if the C_(cc) isset smaller than the value, however, the above effect is lessened.

Embodiment 2

In this embodiment, controlling the discharging of the same power supplyapparatus will be described.

An operation for controlling discharging of the power supply apparatuswill be described with reference to a flow chart shown in FIG. 3. Duringdischarging, the control unit 4 begins detecting of the total voltage ofthe secondary batteries 5. In step 201, the control unit 4 continuesdischarging of the secondary batteries 5 until the total voltage of thesecondary batteries 5 drops to a preset cut-off voltage. The chargedamount of the secondary batteries 5 at this time is assumed as a lowerlimit capacity (C_(d)). In step 202, the control circuit 10 compares avariance (σ_(d)) of the voltages of the units 16-19 detected by thevoltage detecting circuits 6-9 with a preset value (D_(d)). If the σ_(d)value is within the D_(d) value, the control circuit 10 judges thedischarging to be done normally and outputs a signal for finishing thedischarging to the discharging circuit 13. The discharging is thusfinished normally. If the σ_(d) value is over the D_(d) value, however,it replaces the lower limit capacity (C_(d)) with a smaller correctedlower limit capacity (C_(dc)) and continues the discharging. Thedischarging is finished after discharging until the total charged amountof the secondary batteries 5 drops to the C_(dc) value.

It is thus possible to eliminate the memory effect and bring out thefull capacity of each secondary battery as described above by settingthe cut-off voltage lower when the battery is deteriorated, thendischarging.

Although a method for charging in which the upper limit capacity arechanged and a method in which the lower limit capacity is changed indischarging are described separately in the above embodiments, thosemethods may be combined.

It is not necessary to execute the above operation continuously. Afterone correction of the charging or discharging, the variance in capacityamong batteries will almost be eliminated.

The method for charging or discharging in which the upper limit capacityor lower limit capacity is changed while continuously monitoring avariance in capacity among the batteries are described in the aboveembodiments, but it is not necessary to always check the capacity and itmay be possible to change the upper limit or the lower limit as same asabove when the predetermined number of the charge/discharge cycle iscounted employing a control unit provided with a counter for countingthe charge/discharge cycles.

It may be also possible to set in the control circuit a time period inwhich the variance in capacity is expected to become large and to changethe preset value as same as above after the period is passed. At thistime, if the set period is decided by taking the battery temperatureinto consideration, for example, the variance in capacity can besuppressed more effectively. For example, if the battery unit isdisposed under a high temperature or if the temperature in the batteryunit differs among batteries, the set period should be shortened.

Although a voltage is detected from each unit composed of a plurality ofbatteries in the above embodiment, the voltage detecting circuit can beprovided with each of those batteries. Of course, the number of voltagedetecting circuits, as well as the number of batteries in each unit arenot limited only to those described in the above embodiment.

Industrial Applicability

Since the present invention can improve the reliability of a powersupply apparatus, it can also apply to any of general-purpose powersupply apparatuses. Especially, the present invention will be usefulwhen used as a driving power source for an electric vehicle that needs alarge power capacity.

What is claimed is:
 1. A power supply apparatus including a battery unitfor supplying an electric power to a load, and a control unit forstopping charging if the charged amount of said battery unit becomeshigher than a preset value, wherein said battery unit comprises aplurality of secondary batteries connected serially, and said controlunit has a function for changing said preset value of the chargingamount of said battery unit to a larger value when a variance incharging capacity among said secondary batteries becomes large incharging.
 2. The power supply apparatus in accordance with claim 1,wherein the charging rate in the charging is 0.3 C or lower.
 3. Thepower supply apparatus in accordance with claim 1, wherein said controlunit further comprises a function for monitoring the charging capacityof said secondary battery in charging, and changing said preset value ofthe charged amount of the battery unit when the variance in the detectedcharging capacity becomes larger than a predetermined value.
 4. Thepower supply apparatus in accordance with claim 1, wherein said presetvalue of the charging amount of the battery unit is changed when thenumber of charge/discharge cycles of said battery unit reaches apredetermined count.
 5. The power supply apparatus in accordance withclaim 1, wherein said preset value of the charging amount of the batteryunit is changed when the charging is operated a predetermined time afterthe last discharging.
 6. The power supply apparatus in accordance withclaim 1, wherein said control unit further has a function for stoppingdischarging when the discharged amount of said battery unit becomeslarger than a preset value, and a function for changing said presetvalue of discharging amount of the battery unit to a larger value when avariance in remaining capacity among said second batteries indischarging becomes large.
 7. A power supply apparatus including abattery unit for supplying an electric power to a load, and a controlunit for stopping discharging when the discharged amount of said batteryunit becomes larger than a preset value, wherein said battery unitcomprises a plurality of secondary batteries connected serially, andsaid control unit comprises a function for changing said preset value ofdischarging amount of the battery unit to a larger value to continueddischarging when a variance in remaining capacity among said secondbatteries in discharging becomes large.
 8. A power supply apparatus inaccordance with claim 7, wherein said control unit further comprises afunction for monitoring said remaining capacity of the secondarybatteries in discharging, and said preset value of discharging amount ofthe battery unit is changed when a variance in the detected remainingcapacity becomes larger than a predetermined value.
 9. The power supplyapparatus in accordance with claim 7, wherein said preset value of thedischarging amount of the battery unit is changed when the number ofcharge/discharge cycles of said battery unit reaches a predeterminedcount.
 10. The power supply apparatus in accordance with claim 7,wherein said preset value of the discharging amount of the battery unitis changed when the discharging is operated a predetermined time afterthe last discharging.